The present invention relates to video processing, and more particularly to an improved video luminance self keyer that extracts a key signal from an unshaped input video using a clip level that establishes one end of a transition range of the key signal, and uses the key signal and clip level to "shape" the input video in order to avoid double multiplication of the input video by the key signal.
Keying is a common and often used operation in video production. A key consists of three elements: a background video signal, a foreground video signal and a key signal. These three signals typically are input to a mixer, with the key signal being the control input, so that the output of the mixer is foreground video when the key signal equals one, background video when the key signal equals zero, and a proportional mix of the foreground and background videos when the key lies between zero and one. The type of key produced by the operation depends upon the way the key signal is derived. If the key signal is a function of the luminance level of some video signal, it is called a luminance key. If the key signal is a function of the chrominance level of some video signal, it is called a chroma key. If the video used to derive the key is the foreground video used in the mixer, it is called a self key. If the video used to derive the key is unrelated to either the foreground or background videos, it is called an external key.
A circuit typically used to extract a key signal from the luminance portion of a video signal is called a clip and gain circuit. In this circuit the incoming video signal is compared against a threshold level, called the clip level. If the video level is far below the clip level, the key output is typically zero. Likewise if the video level is far above the clip level, the key output is typically one. When the video level is equal to the clip level, the key output is typically one-half. For a region of video levels slightly above and below the clip level the key output varies between zero and one, providing a soft transition from one limit to the other. The width of this transition is determined by the gain of the circuit: high gains cause a small transition region, meaning an abrupt switch at the output of the mixer between the foreground and background videos; and low gains result in broader transition regions, causing the mixer output to make more gradual transitions between the two videos.
The clip and gain levels of the circuit typically are set by an operator for the most pleasing effect, and may be fixed for the duration of a video effect, may vary on a field-by-field basis as part of an in-betweening keyframe sequence, and/or may vary dynamically within a video field as determined by a modulating mask signal. When the clip and gain circuit is used in a self key configuration, a typical problem is that of double multiplication. For video levels within the transition region the key output is a direct function of the input level. This key is then used to mix between the background and the same, i.e., foreground video, input, causing a change in the "shape" of the video. This occurs because the mixer internally multiplies the foreground video by the key signal, where the key signal already is a function of the foreground video. The visual effect of this double multiplication varies depending upon the video content, but typically results in a darkening of the video in the transition region.
What is desired is an improved luminance video self keyer that eliminates the effects of double multiplication.